diff --git a/src/main/extio/Module.mk b/src/main/extio/Module.mk
new file mode 100644
index 0000000..9845d60
--- /dev/null
+++ b/src/main/extio/Module.mk
@@ -0,0 +1,4 @@
+libs		+= extio
+
+src_extio	:= \
+	cmd.c \
diff --git a/src/main/extio/cmd.c b/src/main/extio/cmd.c
new file mode 100644
index 0000000..e392fae
--- /dev/null
+++ b/src/main/extio/cmd.c
@@ -0,0 +1,16 @@
+#include "cmd.h"
+
+uint8_t extio_cmd_checksum(const struct extio_cmd_write *write)
+{
+    const uint8_t *data;
+    uint32_t checksum;
+
+    data = (const uint8_t *) write;
+
+    checksum = 0;
+    checksum += data[0];
+    checksum += data[1];
+    checksum += data[2];
+
+    return (uint8_t) (checksum & 0x7F);
+}
\ No newline at end of file
diff --git a/src/main/extio/cmd.h b/src/main/extio/cmd.h
new file mode 100644
index 0000000..e764705
--- /dev/null
+++ b/src/main/extio/cmd.h
@@ -0,0 +1,49 @@
+#ifndef EXTIO_CMD_H
+#define EXTIO_CMD_H
+
+#include <stdint.h>
+
+#define EXTIO_PAD_LIGHT_MAX_PLAYERS 2
+
+enum extio_status {
+    EXTIO_STATUS_OK = 0x11,
+};
+
+#pragma pack(push, 1)
+
+struct extio_cmd_pad_light {
+    uint8_t unknown : 3;
+    uint8_t right : 1;
+    uint8_t left : 1;
+    uint8_t down : 1;
+    uint8_t up : 1;
+    uint8_t unknown_80 : 1;
+};
+
+struct extio_cmd_write {
+    struct extio_cmd_pad_light pad_lights[EXTIO_PAD_LIGHT_MAX_PLAYERS];
+
+    uint8_t unknown3 : 3;
+    uint8_t sensor_read_mode : 3;
+    uint8_t neons : 1;
+    uint8_t unknown4 : 1;
+
+    uint8_t checksum;
+};
+
+struct extio_cmd_read {
+    uint8_t status;
+};
+
+_Static_assert(
+    sizeof(struct extio_cmd_write) == 4,
+    "struct extio_cmd_write is the wrong size");
+_Static_assert(
+    sizeof(struct extio_cmd_read) == 1,
+    "struct extio_cmd_read is the wrong size");
+
+#pragma pack(pop)
+
+uint8_t extio_cmd_checksum(const struct extio_cmd_write *write);
+
+#endif
\ No newline at end of file